U.S. Defense Secretary Leon E. Panetta says he has seen intelligence reports indicating an al-Qaeda presence in restive Syria.

"Frankly, we don't have very good intelligence as to just exactly what their activities are," he told a joint press conference with Gen. Martin E. Dempsey, Chairman of the Joint Chiefs of Staff, at the Pentagon on Thursday.

The group's presence anywhere is a concern, he said, adding, "we need to continue to do everything we can to determine what kind of influence they are trying to exert there."

At least 55 people were killed and nearly 400 injured in two powerful explosions that rocked the Syrian capital Damascus on Thursday. Syrian officials blamed "foreign-backed terrorists" of carrying out the twin blasts in quick succession during morning rush hour in the Qazaz neighborhood that houses headquarters of the country's military intelligence.

The Pentagon chief acknowledged that the ceasefire nominally in place in Syria as part of U.N. Special Envoy Kofi Annan's peace plan "does not appear to be working."

"We continue to urge Assad to step down, that there must be a change there," Panetta said, and added: "They've lost their legitimacy by the huge number of deaths that are taking place in Syria."

He said the United States would continue to work with other nations to put diplomatic and economic pressure on President Bashar al-Assad to implement political reforms, force him to step down and to try to return Syria to the Syrian people.

The Syrian people's revolt against Assad's regime began in early 2011. Since then, Assad's military has battled rebel forces in several cities. More than 17,000 people, including combatants and civilians, are believed to have been killed in the 14-month-old uprising.

Turning to Yemen, Panetta clarified that DoD's announcement earlier this week that U.S. military personnel have resumed training Yemeni forces does not mean U.S. ground forces are engaged there.

Panetta disclosed that the failed al-Qaeda plot to attack a U.S. airliner using an advanced version of the 2009 underwear bomb was planned to happen in Yemen, which demonstrated that the Yemen-based Al-Qaeda in the Arabian Peninsula (AQAP) remains a threat.

He vowed that "we will go after al-Qaeda wherever they are and wherever they try to hide. And one of the places that they clearly are located is Yemen."

The United States does have operations there, and Yemeni officials have been "very cooperative" in those activities, he told reporters.

"Our operations now are directed with the Yemenis going after al-Qaeda," he said, adding there is "no consideration" of U.S. military ground operations in Yemen.

U.S. efforts to target al-Qaeda leaders -- such as the September 30 U.S. air strike in Yemen that killed terrorist Anwar al-Awlaki -- have been very successful, he noted.

"I think the fact that … we continue to be successful with regards to these kinds of threats is an indication of the effectiveness of the operations that we have there," Panetta said, expressing confidence that "we are making effective progress at going after those specific targets that represent real threats to the United States."